Cooling the World Cup: USA vs Qatar

Only three U.S. World Cup venues can genuinely be described as actively cooled match stadiums: Atlanta, Dallas and Houston.

Cooling the World Cup: USA vs Qatar

Two approaches to conditioning large football stadiums in warm climates.

Only three U.S. World Cup venues can genuinely be described as actively cooled match stadiums: Atlanta, Dallas and Houston. The key point is not concourse air-conditioning, but the ability—when the retractable roof is closed—to condition the spectator bowl and pitch as one large indoor volume. 

Cooling the World Cup: USA vs Qatar

This is effective, but not subtle. The U.S. strategy is to enclose first and cool second. Qatar 2022 used a different logic: architectural form, air movement and local supply to create cooled microclimates around spectators and the pitch. The real comparison is therefore not whether cooling works, but how efficiently, evenly and robustly it delivers comfort, using the least possible energy. 

Is active cooling necessary in football stadiums? 

In June and July, outdoor temperatures in Atlanta, north Texas and Houston can exceed 35°C / 95°F. Atlanta and Dallas have similar absolute humidity, around 17 g/m³, while Houston is closer to 20 g/m³, meaning roughly 20% more moisture in the air. Houston is therefore the most latent-load driven case: the plant must remove water vapour as well as heat. 

For players, high wet-bulb conditions reduce evaporative heat loss, increase cardiovascular strain and can reduce high-intensity running. For spectators, long dwell times, queuing, alcohol consumption and limited personal control turn heat into a crowd-safety issue. Cooling is therefore part of the event strategy, not a luxury. The weakness is that it depends on treating each stadium as a sealed building: roof open, doors open, uncontrolled solar gains or leakage at the retractable roof perimeter all make the cooling task less elegant and more energy intensive. 

Estadio de Atlanta

Atlanta Stadium: integrated, but still a huge, conditioned volume 

Atlanta is the most integrated of the three. Its retractable roof closes the bowl, and the stadium combines daylighting, automated controls, LED lighting and a wider sustainability narrative. It is closest to a whole-building proposition rather than simply a stadium with a very large chiller plant. 

The cooling principle is still conventional: reduce air exchange, enclose the volume, and condition it with high-capacity HVAC. This should give stable operative temperatures once the roof is closed. The limitation is the envelope: a large roof aperture, translucent roof elements and a very high internal volume all carry thermal penalties. It may be efficient by U.S. mega-venue standards, but that is a low bar compared with reducing the conditioned volume or supplying comfort locally. 

Dallas Stadium: effective comfort, blunt energy logic 

Dallas is the clearest expression of the U.S. model: close the roof and operate the stadium as a giant air-conditioned arena. Reported closed-roof conditions are around 22–24°C / 72–75°F even when outdoor temperatures are close to 35°C / 95°F and perceived outdoor conditions approach 41°C / 106°F. Technically, that is a successful comfort outcome.  The cost is energy intensity. Published estimates put Dallas close to 100,000 kWh per match for air-conditioning. In U.S. energy terms, that is about 341 million BTU of electrical input per match before considering plant efficiency and distribution losses. Even allowing for uncertainty, the order of magnitude is telling: comfort is delivered primarily by plant capacity, not by climatic intelligence. The system works, but success is bought mechanically. 

Estadio de Dallas
Estadio de Houston: la advertencia más clara sobre la envolvente

Houston Stadium: the clearest envelope warning 

Houston is the hardest case because heat and humidity combine. With the roof closed, the stadium can act as a conditioned bowl and protect spectators from direct sun and rain. But it also shows why ‘retractable roof’ and ‘thermal envelope’ are not the same thing. The stadium performs well only when the roof and major openings are treated as a serious thermal boundary.  When the roof is open, solar exposure is uneven: south and west areas are generally more protected, while north and east areas are more exposed. This matters because mean radiant temperature can rise even if the air temperature is acceptable. In those conditions, internal shading and strict roof/opening control become part of the cooling strategy, not operational footnotes. Air leakage, roof perimeter gaps, large volumes and solar penetration all reduce the effectiveness of mechanical cooling.

Qatar 2022: targeted microclimate rather than cooled void 

Qatar’s strategy was fundamentally different in the 2022 world cup. It did not attempt to cool an entire stadium void. The principle was spot cooling: supply cool air where people and players actually are, through grilles in the stands and larger pitch-side nozzles. Stadium form, insulation, air recirculation, filtration, wind-tunnel testing and CFD were used to contain a cooled microclimate within a semi-open condition. 

This is a more interesting comfort strategy because it starts from the occupied zone: the spectator’s ankles, the occupied tier, the player and the pitch boundary layer. It also recognises that comfort depends on radiant temperature and air movement, not only dry-bulb temperature. The U.S. approach is simpler and probably more robust operationally: close the roof, set a temperature, run the plant. But it cools a very large volume whether or not each cubic metre contributes to comfort. Qatar’s approach was not free of energy or carbon questions, but conceptually its efficiency argument is stronger because comfort was targeted rather than volume-wide. 

Qatar 2022: microclima dirigido en lugar de enfriar el vacío
Qatar 2022: microclima dirigido en lugar de enfriar el vacío

What monitored data is available? 

The evidence base is thinner than it should be. For the U.S. venues, the best public energy figures are estimates rather than confirmed BMS or utility-meter data: approximately 99,925 kWh per match for Dallas, 97,344 kWh for Houston and 96,020 kWh for Atlanta. Dallas also has a reported internal range of 22–24°C / 72–75°F, but without relative humidity, air speed, radiant temperature or sensor locations, this is not a full comfort dataset. 

For Qatar, the public evidence is technically richer—CFD, wind-tunnel work, targeted supply, air recirculation and reported target conditions around 18–24°C / 64–75°F—but still not equivalent to transparent post-occupancy monitoring. One CFD-based study reported acceptable comfort under external conditions up to 48°C / 118°F and 70% relative humidity, with many zones maintaining thermal neutrality even when cooling load was reduced by about 50%. That supports the design logic, but it remains modelled evidence, not match monitoring. 

A rigorous comparison—kWh per spectator-hour, kWh per comfort-hour, or WBGT reduction per kWh—is therefore not possible from public sources. The available evidence suggests that U.S. closed-roof cooling works, while Qatar’s strategy is conceptually more energy efficient. But neither case has yet published enough monitored data to prove performance transparently. 

Lessons learned 

First, enclosure works only if it is treated seriously. A retractable roof becomes a thermal envelope only when airtightness, solar control, perimeter detailing and operational discipline are adequate. 

Second, comfort must be measured at the occupant, not the thermostat. Air temperature alone is insufficient; radiant temperature, humidity, air speed, stratification and crowd density all matter. 

Third, energy efficiency depends on reducing load before increasing plant size. Qatar’s hierarchy—shade, shape, contain, supply locally, recirculate, then cool—is better environmental design. The U.S. model starts further downstream: close the lid and cool the room. 

The conclusion is simple: Atlanta, Dallas and Houston can host safe, comfortable matches, but they should not be mistaken for low-energy precedents. The best cooling strategy is not a bigger chiller. It is a better solar control, better scheduling (i.e. hosting games at cooler hours of the day) and a better envelope.

Hotel Marcel New Haven: Inside America’s Leading Passive House Hotel at PHN Conference 2026

Discover Hotel Marcel in New Haven, the only Passive House-certified hotel in the US and host of the PHN Conference 2026. A first-hand look at comfort, retrofit design, energy systems, and sustainable hospitality.

Hotel Marcel New Haven: Inside America’s Leading Passive House Hotel at PHN Conference 2026

A first-hand stay at the Passive House Network USA’s 2026 Conference venue.

By Oliver Style, Praxis CEO.

Hotel Marcel New Haven: Inside America’s Leading Passive House Hotel at PHN Conference 2026

Hotel Marcel, New Haven.

Hosting the Passive House Network Conference 2026.

Intro

Hotel Marcel in New Haven is far more than a design hotel: it is one of the most important sustainable retrofit projects in the hospitality sector. Formerly the Armstrong Rubber Company Building, later known as the Pirelli Tire Building, this iconic Brutalist structure was designed by modernist architect Marcel Breuer. After sitting abandoned and unoccupied for nearly two decades, it was purchased by IKEA and partially demolished before being sold on again. In 2019, Bruce Becker—architect, developer, and founder of Becker + Becker—acquired the building and surrounding property from IKEA for $1.2 million. Under the terms of the deal, Becker agreed to remove the remaining asbestos, meet a series of sustainability conditions, and purchase furniture from IKEA. Becker + Becker then set out to transform the building into a carbon-neutral, fully electric hotel that could rely on its own energy generation. The ambition was bold: to create the first Passive House-certified hotel in the United States. Today, Hotel Marcel stands as the world’s first Passivhaus-retrofitted Brutalist hotel and, during the first week of June 2026, it also served as the venue for the Passive House Network USA’s 2026 Conference, where I had the honour of presenting the Mirador de Gracia project: “Certified Senior Care Housing Comes to Catalonia”.

Sensations

One of the most impressive aspects of staying at Hotel Marcel is how clearly you can feel the benefits of high-performance design. The hotel sits immediately next to a major highway, yet once inside it is astonishingly quiet and restful. Triple-glazed windows, excellent airtightness, and—as far as is realistically possible in a retrofit with internal thermal insulation—continuous insulation create a calm, protected indoor environment that feels completely detached from the traffic outside. I stayed there in the first week of June 2026, at the start of summer: the hotel remained exceptionally comfortable, and I had the air conditioning turned off for most of my stay. The indoor air quality was equally noticeable. Unlike many conventional hotels, there was no sense of stale or stuffy air in the morning. Instead, the rooms felt fresh, light, and airy—exactly the kind of comfort Passive House projects promise, but which is rarely so tangible to the guest.

Numbers

Hotel Marcel also makes a strong business case for Passive House in hospitality. During the conference, Bruce Becker explained that Passive House certification has added approximately USD $6 million in asset value to the building. At the same time, hotel operating expenses have been reduced by around USD $450,000 per year. Perhaps most strikingly, retrofitting the building to Passive House standard increased construction costs by only 2%. A large proportion of that uplift was offset through tax credits and utility company rebates, underlining an important point for developers: better buildings are not only more comfortable and lower carbon, but can also be highly attractive financially. “Passivhaus is value engineering!” was the key takeaway of Becker’s inspiring keynote presentation in the Passive House Network 2026 Conference.

HVAC & DHW Systems

The hotel’s heating and cooling are provided by a 3-pipe VRF system with heat recovery, allowing energy to be transferred efficiently across zones with different thermal demands. Ventilation is handled by Swegon energy recovery air handling units with heating and cooling coils. These units modulate the speed of the rotary wheel to recover more or less moisture depending on indoor and outdoor conditions, helping to maintain comfort while reducing energy demand. Domestic hot water is generated by air source heat pumps that charge two storage tanks in series, followed by a final ‘swing’ tank with an electric resistance heater for backup. According to the project team, those swing tank resistance heaters have never needed to come into operation—a telling indication of how effectively the system has been designed.

Control and Electrics

Hotel Marcel’s electrical and control strategy is every bit as forward-looking as its envelope and HVAC design. All LED lighting and internal blackout blinds operate on DC low-voltage power-over-ethernet (PoE) technology. This significantly reduces capital installation costs through fewer AC-to-DC drivers and less cabling, while also delivering operational savings of more than 30% compared with conventional lighting systems. In the guest rooms, the key card system is interfaced with the VRF heating and cooling controls, ensuring that air conditioning cannot be left running when the room is unoccupied. It is a simple but highly effective example of design, controls, and operations working together.

Solar PV & Battery Storage

The hotel’s renewable energy infrastructure is central to its net-zero ambition. Rooftop solar panels and parking-lot canopies together provide approximately 575,000 kWh of solar generation annually. Lithium-ion batteries provide 1.5 MWh of storage, and the site operates through a microgrid that allows the hotel to run in island mode, off the grid. This capacity for resilience, as well as decarbonisation, makes the project especially compelling in a hospitality context where uninterrupted service is essential.

Industrial Kitchen and Laundry Facilities

Behind the scenes, Hotel Marcel applies the same electrification logic to its operational spaces. The kitchen is fully electric, using induction cooking hobs and recirculation hoods that do not require venting to the outside or the conditioning of make-up air. The laundry facilities are also all electric, with heat pump clothes dryers that reduce peak electrical power demand from 128 kW to just 24 kW. The heat released in the laundry room is then absorbed by a heat pump water heater, which contributes to domestic hot water production. It is an elegant example of integrated systems thinking rather than isolated equipment decisions.

Lifts

Even the lifts contribute to the building’s performance strategy. KONE lifts generate power during braking, feeding recovered electricity back into the building’s systems. It is a relatively small measure in overall energy terms, but one that reflects the project’s broader attention to detail.

Food & Beverage

Sustainability at Hotel Marcel extends beyond the building systems into food and beverage operations. The hotel composts all food waste from the restaurant and catering, procures locally sourced ingredients and sustainably produced offerings, uses induction banquet equipment with no sterno usage, and opts for linenless tables in order to reduce the energy consumption associated with laundry. These choices show that environmental performance in hospitality depends as much on operational culture as on the building itself.

Recyclable & Healthy Products

The hotel’s approach to products and housekeeping is equally consistent. No single-use plastics are used for events, dining, or guest-room bathroom amenities. Environmentally friendly cleaning products are used throughout the building, and linens and towels are washed on request rather than nightly. These measures may sound simple, but together they reinforce the hotel’s wider commitment to healthier, lower-impact hospitality.

Service

What also stood out during my stay was the quality of service. The staff were warm, friendly, and clearly passionate about the hotel and the project as a whole. During the building tour, that pride was especially noticeable: they were enthusiastic in explaining what had been implemented, why it matters, and how the hotel operates differently from a conventional property. That human element—people who genuinely believe in the story they are telling—adds enormous value to the guest experience.

Hotel Marcel, New Haven

Certifications

Hotel Marcel’s performance has been recognised through an exceptional set of sustainability certifications. It has achieved LEED Platinum Certification, making it one of only 10 hotels in the United States to be certified LEED Platinum under the US Green Building Council’s standards, with points earned across categories such as water efficiency, energy efficiency, and integrated design process. It also holds Passive House Certification, making it the only Passive House-certified hotel in the US and the largest in the world, with a design that reduces energy use by as much as 80%. Net-zero certification is still pending, but the project is on track to become the nation’s first net-zero hotel through the elimination of fossil fuel usage.

Final Thoughts

Hotel Marcel is one of those rare buildings that succeeds on every level. Architecturally, it preserves and reinterprets a major piece of modernist heritage. Environmentally, it demonstrates that even difficult existing buildings can be transformed into high-performance assets. Commercially, it shows that Passive House can make sense in hospitality. And from the guest perspective, it is simply a very comfortable place to stay. Experiencing it during the Passive House Network USA’s 2026 Conference made that even clearer: the building was not just the venue, but part of the argument—living proof that the future of sustainable hospitality is already here.

Classroom overheating in Barcelona: HVAC simulation study

EnergyPlus simulation study on classroom overheating in Barcelona, thermal comfort, solar orientation and HVAC strategies for schools.

Classroom overheating in Barcelona: HVAC simulation study

Prevención del sobrecalentamiento en aulas: un estudio de simulación termodinámica en Barcelona
Plano aula

Introduction: why classroom overheating matters

As climate change intensifies summer heatwaves, preventing classroom overheating has become a critical design challenge for educational buildings in Mediterranean cities such as Barcelona. Schools must provide healthy, comfortable indoor conditions while also reducing energy consumption and operational complexity.

This article presents a thermodynamic building simulation study assessing overheating risk in two primary-school classrooms in Barcelona. The analysis uses DesignBuilder with the EnergyPlus calculation engine to evaluate whether a centralised 100% outdoor air HVAC system can maintain thermal comfort during school hours from May to September.

The study is relevant for architects, engineers, school operators and public authorities seeking evidence-based strategies for thermal comfort in schools, especially where cooling is delivered through a central air-handling unit rather than room-by-room terminal units.

* Schools shut in Spain from the end of June until September. However, for the simulations, July and August were included in the simulations, to have results under more challenging climate conditions. This was also done as the climate file is based on historical data. Currently, weather conditions that typically occurred in July now often occur in May.

Study objectives

The simulation was designed to answer three practical questions about classroom overheating, supply-air temperature and the impact of solar orientation.

  • How do east- and west-facing classrooms behave differently due to solar exposure, and what does this mean for HVAC control?
  • How many teaching hours exceed an operative temperature of 27°C despite cooled air being supplied at 14°C?
  • What relationship between outdoor temperature and supply-air temperature helps avoid both overheating and overcooling?

Simulation tools and building model

The dynamic simulation was carried out in DesignBuilder using the EnergyPlus engine. The model represents two identical classrooms located on the third floor of a primary school in Barcelona: one facing east and one facing west, with a north–south corridor between them.

The Barcelona–Airport IWEC II weather file developed by ASHRAE was used for the climate assumptions. July and August were included in the simulation period even though schools in Spain typically close during summer holidays, because these months provide a more demanding stress test for the system and help evaluate future climate resilience.

Internal floors and partitions were modelled as adiabatic. The envelope parameters comply with the minimum requirements of Spain’s CTE HE1 code for Climate Zone C. Windows are double glazed with Ug = 1.80 W/m²K, 79% visible transmittance and a solar factor of 59%. The external wall has U = 0.49 W/m²K, the roof has U = 0.40 W/m²K and air permeability is n50 = 3 ach.

Each classroom includes fixed external shading devices with a 50% reduction factor, representing expanded metal mesh shading. This shading reduces solar gains but does not eliminate the impact of orientation.

Internal loads and HVAC configuration

Each classroom has a floor area of 60 m² and is occupied by 31 pupils and one adult teacher during school hours. Lighting and equipment loads operate during teaching hours, Monday to Friday from 08:00 to 18:00, although lighting is assumed to be off during summer.

Ventilation is provided by a central air-handling unit supplying 100% outdoor air at 45 m³/h per person, equivalent to 1,395 m³/h per classroom. The system includes heat recovery with 79% sensible efficiency and 62% latent efficiency.

Cooling is provided by an air-to-water heat pump producing chilled water at 7°C for the AHU cooling coil. During summer school hours, the supply air temperature is assumed to be a constant 14°C at classroom supply level. In practice, duct heat gains and pressure losses may mean that the delivered supply air temperature and airflow differ from the simulation assumption. For May and September, a dual-setpoint control strategy is used to reduce overcooling: when outdoor temperature is below 16°C, supply air is delivered at 20°C; when outdoor temperature rises above 17°C, supply air is delivered at 14°C.

Internatl confort range

Thermal comfort criteria

Thermal comfort was assessed using operative temperature thresholds during school hours only. The comfort band was defined as 22°C to 27°C, with indoor relative humidity targets between 30% and 60% at 26°C. Conditions below 22°C were classified as too cold, while conditions above 27°C were classified as too hot.

Using operative temperature rather than air temperature alone provides a more representative assessment of how occupants experience the room, because it accounts for both air temperature and radiant temperature from surrounding surfaces.

Key results: overheating and overcooling risk

External Climate Overview (08:00–18:00, Mon–Fri)

The analysis shows that the central HVAC system generally maintains classroom comfort for most teaching hours. However, the results also demonstrate that orientation, solar gains and supply-air control are decisive factors in determining real comfort performance.

Key Results
Overheating Risk – Summary (% of teaching hours > 27 °C)

The east-facing classroom experiences the highest overheating risk, particularly in July and September, because morning solar gains coincide with early occupancy and rising outdoor temperatures. In July, the east classroom exceeds 27°C during approximately 7% of teaching hours despite the supply air being delivered at 14°C.

Overcooling Risk – Summary (% of teaching hours < 22 °C)

The west-facing classroom performs slightly better in terms of overheating, with only a small proportion of hours above 27°C in September. However, it is more exposed to overcooling in the morning during shoulder months, because delayed afternoon solar exposure does not compensate for early cool supply air.

This finding illustrates a common HVAC design challenge in schools: a single central control strategy can perform adequately on average while still creating different comfort outcomes across zones with different orientations.

Why orientation matters in classroom comfort

Solar orientation plays a significant role in overheating risk. East-facing classrooms receive direct solar radiation in the morning, when pupils arrive and classrooms are already occupied. This can quickly raise operative temperatures before the cooling system has fully compensated for the solar load.

West-facing classrooms receive more delayed solar radiation in the afternoon. This can reduce early-morning overheating risk but may create a different comfort profile later in the day. In the simulation, the west classroom was also more prone to mild overcooling during May and September.

For school buildings in Barcelona and other Mediterranean climates, façade orientation should therefore be considered alongside HVAC design. Identical classrooms can require different control strategies even when they have the same area, occupancy and envelope specification.

Recommended strategies to prevent classroom overheating

The simulation confirms that HVAC control is important, but it should be combined with passive design measures to create resilient school buildings. The most effective design approach is to reduce heat gains first, then use efficient mechanical systems to manage the remaining cooling load.

  • Improve external shading, especially on east and west façades where low-angle solar radiation is difficult to control.
  • Optimise glazing specifications, particularly solar factor, visible transmittance and frame performance.
  • Improve airtightness and insulation to reduce unwanted heat transfer and improve system predictability.
  • Reduce internal heat gains from lighting, equipment and operational schedules.
  • Consider night ventilation or free-cooling strategies where climate, acoustics and security conditions allow.
  • Commission airflow rates and supply-air temperatures carefully at classroom level rather than only at the AHU.

HVAC design lessons for schools

For 100% outdoor air systems serving multiple classrooms, ductwork design and commissioning are critical. Long duct runs can introduce pressure losses and heat gains, reducing airflow and increasing supply-air temperature at the classroom grilles. These effects can be especially important when the design relies on cool supply air to manage sensible loads.

The study also highlights the value of simple but intelligent control logic. A dual-setpoint strategy based on outdoor temperature can reduce overcooling in shoulder months without adding excessive complexity. However, zone-level monitoring or balancing may still be needed when classrooms have very different orientation or solar exposure.

Energy simulation with tools such as EnergyPlus and DesignBuilder allows design teams to test these strategies before construction or refurbishment. This supports better decisions, reduces performance gaps and helps align thermal comfort with energy-efficiency objectives.

Conclusions

The results show that a centralised 100% outdoor air HVAC system can generally maintain acceptable comfort in classrooms in Barcelona, but overheating risk is not eliminated. The east-facing classroom remains more vulnerable to overheating, especially in July, while the west-facing classroom shows a greater tendency toward mild overcooling during shoulder-season mornings.

Preventing classroom overheating requires a combined strategy: accurate energy simulation, façade-specific passive design, robust HVAC controls and careful commissioning. For Mediterranean school buildings facing warmer and more variable climate conditions, this integrated approach is essential for health, comfort, educational performance and energy efficiency.

FAQ: classroom overheating and thermal comfort

What causes classroom overheating?
Classroom overheating is caused by a combination of high outdoor temperatures, solar gains through windows, internal heat gains from pupils and equipment, insufficient shading and poorly matched HVAC control.

How can overheating in schools be prevented?
The most effective approach combines passive measures such as external shading, better glazing and reduced internal gains with HVAC strategies such as adaptive supply-air temperature control and careful commissioning.

What is a comfortable classroom temperature?
In this study, the comfort range was defined as an operative temperature between 22°C and 27°C during occupied school hours.

Why use EnergyPlus for school overheating studies?
EnergyPlus enables hourly dynamic simulation of weather, occupancy, solar gains, envelope performance and HVAC operation, making it suitable for evaluating overheating risk before decisions are implemented on site.

Can Naiades: first winter in our Passivhaus…warmth, comfort and tiny energy bills

First winter in our Passivhaus: The house has felt warm, calm and incredibly comfortable throughout the winter.

Can Naiades: first winter in our Passivhaus…warmth, comfort and tiny energy bills

By Oliver Style, Praxis CEO

I’ve lived in a few cool-temperate climates over the years – the UK, Switzerland, northern France – places where you’d expect winter to feel like winter. But, weirdly enough, I don’t think I’d ever really felt the cold at home as much as when I first moved to Barcelona in 2010 and installed myself in a flat in the historic city centre. Not because Barcelona is especially cold. It isn’t at all. But because many homes aren’t fit for the (thankfully!) short Mediterranean winter, and- in the case of the flat I moved in to- didn’t have the luxuries of central heating.

So, having got through our first summer in Can Naiades — our new Passivhaus — with flying colours, the next big question was: what would winter be like?

Would the house stay warm? Would the real energy consumption match the PHPP model? Would the solar PV and battery still pull their weight at the time of year when solar generation is at its lowest and energy demand is at its highest? Let’s have a look…

Can Naiades
Can Naiades

Nice and toasty

The short version is: it has been absolutely lovely.

The house has felt warm, calm and incredibly comfortable throughout the winter. Not “warm” in that slightly aggressive way you get when a radiator is blasting away in one corner of the room while your feet are still cold. Just evenly, quietly, consistently warm.

Temperatures have been stable and homogeneous throughout the house, with no noticeable cold spots, no draughts, and no cold air pouring in around windows, doors or junctions. That might sound like a small thing, but when you’ve lived in enough leaky buildings, the absence of discomfort becomes a kind of luxury.

There’s also something very particular about the quality of the air in a Passivhaus in winter. Because the house is airtight and ventilated continuously with heat recovery, we’re not relying on random infiltration through cracks and gaps to provide “fresh” air. The ventilation system just gets on with it: extracting stale, humid air from the kitchen and bathrooms, supplying filtered fresh air to the bedrooms, office and living spaces, and recovering heat from the outgoing air in the process.

The result is a house that feels fresh without feeling cold. That’s the magic trick. And then there’s the other great thing: no mould and no condensation. None. Zero.

This is the first home I’ve lived in, in Catalonia, where winter has not meant some combination of wet window frames, condensation on glass, suspiciously dark corners, or the occasional nasty surprise behind a wardrobe. In previous homes, condensation and mould felt like something you just had to manage: ventilate more, heat more, wipe it down, paint it over, move the furniture away from the wall, hope for the best.

Here, it just hasn’t happened. That’s not an accident. Warm internal surface temperatures, good insulation, reduced thermal bridges, airtightness, and continuous mechanical ventilation all work together. In practice, the experience is wonderfully uneventful: walls stay warm, indoor humidity is controlled, the air stays fresh, and nothing goes furry.

Fantastic!

The numbers

Of course, comfort is the most important thing. Buildings are for people, not spreadsheets. But the numbers matter too — especially if we want to show that low-energy, high-comfort buildings work not just in theory, but in real life, with real families, real weather, real cooking, real showers, real washing machines, and real life generally getting in the way.

So how did Can Naiades perform?

Really well. Over the winter period, the total measured energy consumption of the house was only 3% higher than the values predicted by the PHPP energy model. That’s a pretty remarkable result, given that PHPP is a design tool and real life is…well, real life, and consistently unpredictable.

There are always differences between modelling and measured performance: occupant behaviour, set-point temperatures, appliance use, hot water consumption, weather variations, commissioning, controls, and the million small things that happen once a building is occupied. So, to be within 3% of the predicted value is a very good sign that the design assumptions, construction quality and installed systems are all broadly doing what they were supposed to do.

Even better, despite winter being the worst time of year for solar generation — short days, lower sun angles, more cloud, and higher household demand — we were still 68% self-sufficient between November and February, using electricity generated by our solar PV panels and stored in the battery.

That really changes your relationship with energy. You become much more aware of when the sun is shining, when the battery is full, when it makes sense to run the washing machine, and how little energy the house actually needs to stay comfortable. It’s not about living with less comfort. It’s about getting more comfort from much less energy.

Compared with our previous flat, the difference is stark. This winter, we spent 79% less on energy bills and used 93% less energy.

Ninety-three percent less energy!

That number still makes me stop and look at it twice. Because this isn’t a smaller, colder, more miserable house. It’s the opposite: it’s bigger, warmer, healthier, quieter and more comfortable. We’re not saving energy by putting up with discomfort. We’re saving energy because the building fabric does most of the work before the mechanical systems even need to get involved.

That, for me, is the key point.

The house doesn’t need much heating because it doesn’t lose much heat and because the sun does most of the heating. The windows don’t feel cold because they’re high-performance and properly installed. The air doesn’t feel stale because the ventilation system is doing its job. The indoor temperature doesn’t swing all over the place because the envelope is insulated, airtight and carefully designed. The systems can be small because the demand is small.

It’s all very boring, really. And that’s exactly how it should be.

Can Naiades: First Winter PHPP vs. measured energy comsumption
Can Naiades: First Winter. Measured solar PV production
Can Naiades: First Winter. Real solor PV generation vs. real energy consumption

The best kWh…

After our first summer, I wrote that living in a Passivhaus in a Mediterranean heat wave felt like a dream come true. After our first winter, I’d say the same thing again — only with a jumper I didn’t really need.

Can Naiades has been warm, fresh, dry and comfortable, while using a tiny amount of energy. The PHPP model has proven to be very close to measured reality. The PV and battery have provided a surprisingly high level of self-sufficiency, even in winter. And our energy bills have dropped dramatically compared with our previous home.

But beyond the technical satisfaction, there’s a broader reflection.

We often talk about energy in terms of production: more renewables, more generation, more infrastructure, more supply. And of course, we need all of that. But living in this house is a daily reminder that the best kWh is still the one you don’t consume.

That truth resonates every time another war breaks out, another geopolitical crisis sends energy prices sky high, or another family has to choose between heating their home properly and paying the rest of the bills.

Reducing demand is not boring. It’s resilience. It’s comfort. It’s climate action. It’s protection against volatile energy prices. And, at the most basic level, it means living in a home that feels good.

After one summer and one winter in Can Naiades, I can confirm: Passivhaus works. And it works beautifully.

Check out our previous articles about the project:

Thank you to the following people and companies for their support:

Passivhaus Homes in Tordera: BYKO completes the Guifré el Pilós development certified by Praxis

Meta description: BYKO delivers three Passivhaus homes in Tordera certified by Praxis: Zehnder heat recovery ventilation, aerothermal DHW, ETICS and 2D timber modules.

Passivhaus Homes in Tordera: BYKO completes the Guifré el Pilós development certified by Praxis

Viviendas Passivhaus en Tordera: BYKO culmina la promoción Guifré el Pilós certificada por Praxis

In Tordera (Barcelona), BYKO has completed the Guifré el Pilós development: a set of three terraced homes designed and built to meet the Passivhaus – Low Energy Building standard, with certification carried out by Praxis Resilient Buildings. The project was developed in collaboration with La Llotja Arquitectes and is located in Tordera, province of Barcelona, Catalonia, Spain.

Architecture

Each home is arranged over two floors with a treated floor area close to 100 m² and a highly functional layout: open‑plan kitchen-living‑dining room, and a full bathroom on the ground floor; and on the first floor: two single bedrooms, a master bedroom with en‑suite, an additional bathroom, a utility/plant room and a study area. Private front and rear patios complete the programme.

Construction system, structure and 2D prefabrication

The construction system uses 2D industrialised lightweight timber‑frame modules for walls and roof, reducing on‑site time and ensuring precision—key to achieving Passivhaus‑level of air‑tightness and thermal‑bridge control. Modules were delivered with the air‑tight layer (FINSA Superpan timber particle board) and external thermal insulation (pre‑render) already installed. Windows are PVC with triple low‑e, solar‑control glazing, and motorised shutters for summer shading.

Building services, ventilation and indoor air quality

  • Balanced mechanical ventilation with heat and moisture recovery (Zehnder Climos 200) for continuous fresh air, minimal thermal loss and effective particle filtration.
  • Space conditioning via air‑to‑air heat pump with ducted indoor units, sized for the building’s low heating and cooling loads.
  • Domestic hot water via an air-to-water heat pump hot water unit with a 190‑L tank; seasonal efficiency for DHW of SCOP = 3.52.
  • Loxone monitoring and controls (ventilation, HVAC, DHW and general electricity) for optimisation and maintenance.
  • Optional rooftop PV to further improve the energy balance.

Passive design, air‑tightness and thermal bridges

As part of the Passivhaus certification, Praxis verified air‑tightness (n₅₀) via blower‑door testing, the elimination of thermal bridges at envelope junctions and openings, active solar protection with motorised shutters, and proper commissioning of the MVHR system. These measures cut heating and cooling demand and deliver 24/7 comfort with very low energy use.

360º comfort

  • Homogeneous indoor temperatures without drafts or noticeable stratification.
  • Consistently healthy indoor air quality thanks to MVHR with filtration (CO₂ and VOCs kept in check).
  • Low noise levels due to the high‑performance envelope and windows.

Voices from the project

“With Guifré el Pilós we reaffirm our commitment to ultra‑low‑energy, high‑comfort homes, bringing the Passivhaus standard to families who value quality, health and efficiency,” says Ismael Fernández, developer and builder at BYKO.

“Passivhaus certification rigorously validates real‑world building performance. At Guifré el Pilós, we paid careful attention to air‑tightness, thermal‑bridge control, high‑efficiency ventilation and summer performance, to ensure year‑round comfort,” says Oliver Style, Passivhaus certifier at Praxis Resilient Buildings.

Can Naiades: real-life performance of a mechanical ventilation unit with heat and moisture recovery

Mechanical ventilation with heat recovery (or MVHR) is a key component of passive houses. This type of system consists of a ventilation unit with a heat exchanger, which transfers the energy from the air being taken out of the house

Can Naiades: real-life performance of a mechanical ventilation unit with heat and moisture recovery

Mechanical ventilation with heat recovery.

Can Naiades: rendimiento real de una unidad de ventilación mecánica con recuperación de calor y humedad

Mechanical ventilation with heat recovery (or MVHR) is a key component of passive houses. This type of system consists of a ventilation unit with a heat exchanger (and humidity exchanger if the unit is enthalpic), which transfers the energy from the air being taken out of the house, to the fresh air being supplied from outside (in summer this process operates in reverse). In this way, in winter, supply air is preheated, using only a small amount of electricity for the fans. In summer, when the indoor air is cooler than the outdoor air, the process is reversed: the air supplied to the interior is cooled.

The ventilation unit also allows the incoming air to be filtered, significantly improving air quality and reducing the amount of dust and other airborne particles.

The system operates continuously, at low airflow rates and without noise. Air is extracted from wet rooms such as bathrooms and the kitchen (eliminating the need to install separate extractor fans with roof ducts), and fresh air is supplied to dry rooms such as living rooms, dining rooms, and bedrooms.

La ventilación de doble flujo
Fuente: BPC Ventilation

Real‑life performance at Can Naiades

We have evaluated the real-life performance of a Zehnder ComfoAir Q450 ERV unit, installed by Fontalgar Instalaciones in the Can Naiades house, working at a flow rate of 200 m3/h. To calculate the actual performance of the machine, we’ve used the Passive House Institute’s efficiency equation:

Fórmula

Where:

Fórmula explicación

The performance data of the Passivhaus Certificate is shown below (full data here). The heat recovery rate according to the certificate is 83%:

Los datos de rendimiento del certificado de componente Passivhaus para invierno
Source: Passivhaus Institute

The Zehnder ComfoAir Q450 ERV model has not yet been certified for cooling performance. However, the ComfoAir Q350 ERV (being a very similar machine), has a cooling recovery certificate (complete data here). The cooling recovery rate in summer according to the certificate is 81%:

El modelo Zehnder ComfoAir Q450 ERV
Source: Passivhaus Institute

In both cases, the electrical consumption of the equipment is certified at 0.21 Wh/m3. In summary, the certified values are:

the certified values

Field inputs and real-life performance

Temperature data were taken for a cold day in winter, and a hot day in summer. The temperatures in each case are as follows:

The temperatures in each case winter

Winter

Fórmula invierno
The temperatures in each case summer

Summer

Fórmula verano

The electric term was calculated as follows:

Fórmula

The calculation of heat recovery efficiency in winter and summer is shown below:

Efficiency in winter:

Efficiency in winter

Efficiency in summer:

Efficiency in summer

Finally, electricity consumption, measured for 6 months of 2025 (the data available at the time of writing), was 150 kWh, shown below:

consumo eléctrico, medido durante 6 meses del año 2025

At a flow rate of 200 m³/h, for 4416 hours (June-December inclusive), that’s a total volume of 883,200 m³. The average specific fan energy consumption is therefore Wh/m³ = 883,200 ÷ 150,000​= 0.1698 Wh/m³.

In summary, the certified values compared to the measured values are as follows:

the certified values

Conclusions

Regarding heat / cooling recovery, the comparison shows that the installed unit has a real-life performance very close to the certified values, both in winter and summer. As for electricity consumption, the measured value is lower than the value of the certificate.

For the moment analysed in winter, the MVHR unit pre-heats the supply air from -1.5ºC to 19.9 ºC, with a ΔT = 21.4 ºC and a COP = 42 (recovered heating power ÷ electrical power input). This translates into significant savings in space heating energy consumption.

We’d like to thank Fontalgar Instalaciones and Zehnder Group Ibérica for their support on this project.

Fontalgar
Zehnder

Glaser vs WUFI: Comparative Hygrothermal Analysis of Interior Insulation on Solid Brick Walls in Two Climates

We know that a deep energy retrofits can be delicate, so the question often arises: what tools can I use for the hygrothermal analysis of potential moisture damage?

Glaser vs WUFI: Comparative Hygrothermal Analysis of Interior Insulation on Solid Brick Walls in Two Climates

We know that a deep energy retrofits can be delicate, so the question often arises: what tools can I use for the hygrothermal analysis of potential moisture damage?

Glaser vs. WUFI: ¿Qué método es fiable para analizar patologías por humedad en muros macizos con aislamiento interior?

In this article, we present a comparison between the simplified Glaser calculation method, as specified in ISO 13788 [1], and dynamic hygrothermal simulation with WUFI Pro 1D [2], in accordance with EN 15026 [3], in two Spanish climates: Barcelona and Burgos. The case study looks at a historic solid brick wall, unrendered, with interior insulation, where we analyse the relative humidity on the interior face of the existing wall, and compare the results from each calculation method. Although ISO 13788 makes clear what the limitations of the Glaser method are, and that it should not be used in cases like this, in practice it is still widely used among building professionals. The results underline the limitations of the simplified method for analysing moisture transport in solid walls with interior insulation.

Introduction

A whole‑building deep energy retrofit radically changes the hygrothermal response of the building envelope. What if I can only install interior insulation on a solid brick wall, exposed to driving rain? Will there be condensation and moisture problems? What tools can I use to analyse the risk? One of the most common tools is the Glaser method, included in ISO 13788. But how reliable are its results?
Let’s look at the results of a comparative study between the Glaser method and a dynamic hygrothermal simulation with WUFI Pro 1D, for a solid brick wall with interior insulation, in the climates of Burgos and Barcelona, Spain.

Figura 1: Cálculo WUFI (izq.) vs. Glaser (der.)
Figure 1: WUFI (left) vs. Glaser (right).

What tools can I use for a hygrothermal risk analysis of moisture damage in retrofit projects?

The best‑known calculation method is Glaser, covered by ISO 13788 and developed in 1958 for analysing lightweight assemblies. It is a simplified calculation method, based on monthly average indoor and outdoor temperatures and relative humidity. It assumes:

  • Steady‑state heat transfer
  • Moisture transfer only by vapor diffusion
  • Materials are completely dry at the start

The calculation determines whether there are critical points of condensation over one year, neglecting the following physical processes:

  • Variation of hygrothermal properties of materials due to their moisture content
  • Latent heat absorption and release
  • Capillary suction and liquid transport within materials
  • Airflow through the building element
  • The hygroscopic capacity of materials

ISO 13788 states that the method is valid only for building elements where these effects are negligible. So it shouldn’t be used to analyse massive assemblies with interior or exterior insulation, or for elements exposed to rain or subject to freeze–thaw cycles. Spain’s CTE DB‑HE building regulations also explicitly states this premise. Nevertheless, the Glaser method is often used incorrectly in practice.

By contrast, dynamic hygrothermal calculation via numerical simulation—described in EN 15026 and implemented in tools like WUFI and Delphin—addresses Glaser’s limitations through an hourly numerical analysis that considers all the above physical processes, realistic boundary conditions, and initial moisture conditions in materials, reflecting real‑world scenarios in retrofit or new construction.

Comparison: Glaser vs. WUFI — Solid Brick Wall with Interior Insulation

Below are the results of a comparative study between Glaser and WUFI, for the climates of Barcelona and Burgos. A 5‑cm interior thermal insulation layer is applied. A second variant with a vapor barrier on the warm side of the insulation is also studied.

The brick wall is 29 cm thick. Of the 29 cm, 80% is brick and 20% is lime mortar. The one‑dimensional section is subdivided to reflect this brick‑mortar ratio, according to the data in Figure 2, following the methodology of Little et al. [4]. Simulations in WUFI were initialized with materials at a moisture content corresponding to 80% RH, at 20°C. Simulations were run for 10 years, starting in October. The WUFI results presented in the comparison with Glaser correspond to year 10. The wall orientation in the WUFI calculations is north, with a rainwater penetration coefficient of 70%. Basic hygrothermal properties of the sample materials are shown in Figure 2.

Figure 2: Basic hygrothermal properties of materials
Figure 2: Basic hygrothermal properties of materials

To compare WUFI results with the monthly Glaser method (whose results do not have an hourly resolution), monthly average values of temperature and relative humidity were extracted from the hourly WUFI outputs.

Results

Figure 3 shows the results for the Barcelona climate with 5 cm of interior insulation. In January, Glaser yields temperatures 16% higher than the dynamic calculation and relative humidity 14% lower.

Figure 3: Results for Barcelona, wall with 5 cm interior insulation
Figure 3: Results for Barcelona, wall with 5 cm interior insulation

Figure 4 shows the results for the Burgos climate. In January, Glaser yields temperatures 22% higher than the dynamic WUFI calculation and relative humidity 4% lower.

Figure 4: Results for Burgos, wall with 5 cm interior insulation
Figure 4: Results for Burgos, wall with 5 cm interior insulation

Figure 5 shows the results for Barcelona with a vapor barrier installed between the interior insulation and the gypsum plasterboard. In January, Glaser yields temperatures 16% higher than the dynamic WUFI calculation and relative humidity 81% lower. The Glaser method indicates no risk of moisture damage, with a maximum RH of 67%, whereas WUFI indicates a mean RH of 99%, implying risk of moisture‑related pathologies.

Figure 5: Results for Barcelona, wall with vapor barrier + 5 cm interior insulation
Figure 5: Results for Barcelona, wall with vapor barrier + 5 cm interior insulation

Figure 6 shows the results for the Burgos climate. The trend is identical: the Glaser results yield much lower relative humidity values than the dynamic calculation with WUFI.

Figure 6: Results for Burgos, wall with vapor barrier + 5 cm interior insulation
Figure 6: Results for Burgos, wall with vapor barrier + 5 cm interior insulation

Conclusions

In Barcelona without a vapor barrier, the relative humidity results on the interior face of the existing wall are 2% to 28% lower with the Glaser method than the dynamic WUFI results. In Burgos, the difference ranges between 2% higher and 26% lower.

For the wall with a vapor barrier, the difference is much more pronounced: from 48% to 81% lower in Barcelona, and from 62% to 116% in Burgos.

The results indicate that the Glaser method described in UNE‑ISO 13788 is not suitable for analysing moisture transfer in unrendered solid walls exposed to rain with interior insulation. The large discrepancy between the results may lead to incorrect hygrothermal design and potential interstitial moisture damage.

For situations like this, that are hygrothermally sensitive, we recommend carrying out a dynamic calculation and/or consulting a specialist. We also recommend extending the study to analyse the water content of materials and the effect of air infiltration/exfiltration (beyond an analysis of relative humidity on the interior face of the wall). In addition, we recommend in‑situ testing to determine the liquid transport coefficient of a historic solid brick wall, since its hygrothermal behaviour can vary widely.

References

[1] EN ISO 13788:2016. Hygrothermal performance of building components and building elements — Internal surface temperature to avoid critical surface humidity and interstitial condensation — Calculation methods (ISO 13788:2012).

[2] WUFI (Wärme Und Feuchte Instationär): dynamic hygrothermal simulation software for analysing heat and moisture transport in building components, developed by the Fraunhofer Institute, Germany.

[3] EN 15026:2007. Hygrothermal performance of building components and building elements — Assessment of moisture transfer by numerical simulation.

[4] Joseph Little, Carolina Ferraro & Beñat Arregi (2015). Assessing risks in insulation retrofits using hygrothermal software tools. Heat and moisture transport in internally insulated stone walls. Historic Environment Scotland Technical Paper 15, Second Edition, 2015, Edinburgh, Scotland.

[5] ASHRAE 160‑2016. Standard 160‑2016 — Criteria for Moisture‑Control Design Analysis in Buildings (ANSI Approved).

Passivhaus EnerPHit certification for existing buildings: What is it and how to achieve it?

As the demand for energy-efficient buildings continues to grow, the retrofit of existing buildings to meet modern efficiency standards has become increasingly important.

Passivhaus EnerPHit certification for existing buildings: What is it and how to achieve it?

As the demand for energy-efficient buildings continues to grow, the retrofit of existing buildings to meet modern efficiency standards has become increasingly important.

EnerPHit: certificación Passivhaus para rehabilitación de edificios existentes. ¿Qué es y cómo conseguirla?

Passivhaus EnerPHit certification provides a rigorous and effective framework for deep energy retrofits, ensuring optimal energy performance and comfort. This article outlines the pathways to achieving EnerPHit certification, its advantages, and considerations for partial renovations.

Pathways for achieving EnerPHit certification

There are two primary pathways to achieving EnerPHit certification: performance-based and prescriptive, together with common requirements that apply to both. Let’s take a look at each one.

1. EnerPHit Energy Demand Method

This performance-based approach is similar to Passivhaus certification for new builds but with slightly relaxed heating and cooling demand requirements, adjusted for the seven global climate zones defined by the Passivhaus Institute, shown in Figure 2.

Figura 2: Criterios de demanda energética EnerPHit  (Fuente: Passivhaus Institute, Criterios para edificios,  Versión 10c del 20/09/2024)
Figura 2: Criterios de demanda energética EnerPHit (Fuente: Passivhaus Institute, Criterios para edificios, Versión 10c del 20/09/2024)

2. EnerPHit Building Component Method

This prescriptive approach sets maximum thermal transmittance values (“U-values”) for each building element, requires control of solar gains, and establishes minimum performance requirements for mechanical ventilation with heat or moisture recovery, depending on the climate zone (Figure 4). The aim is to ensure that the retrofit is highly energy-efficient and safe with respect to moisture-related pathologies.

Figure 4: EnerPHit Building Component Method (Source: Passivhaus Institute, Criteria for Buildings, Version 10c as of 1/20/2023)
Figure 4: EnerPHit Building Component Method (Source: Passivhaus Institute, Criteria for Buildings, Version 10c as of 1/20/2023)
Figura 5: Rehabilitación EnerPHit por Componentes, Sant Cugat del Vallès, Marcove (Fuente: Jose Hevia)
Figure 5: Single-family home certified to EnerPHit standard, Component Method, Marcove, Sant Cugat, Catalonia (Source: Jose Hevia)

Common requirements (for both pathways)

For both pathways, there are common requirements. Regarding the level of air infiltration, the maximum allowed value in the airtightness (Blower Door) test is n50 = 1.0 air changes per hour (instead of n50 = 0.6 ach required by Passivhaus for new builds). Additionally, the total renewable primary energy consumption of the building is limited, depending on whether it is certified as EnerPHit Classic, Plus, or Premium (Plus and Premium include renewable energy generation), as shown in Figure 6. Each certification class has its respective seal, shown in Figure 7.

Figure 6: General EnerPHit criteria (irrespective of the method) (Source: Passivhaus Institute, Criteria for Buildings, Version 10c as of 1/20/2023)
Figure 6: General EnerPHit criteria (irrespective of the method) (Source: Passivhaus Institute, Criteria for Buildings, Version 10c as of 1/20/2023)
EnerPHit Classic
EnerPHit Plus
EnerPHit Premium

Figure 7: EnerPHit Classic, Plus y Premium seals

Advantages of EnerPHit certification

Pursuing EnerPHit certification provides numerous benefits:

  • Holistic deep energy retrofit: Ensures comprehensive upgrades that prevent moisture damage associated with partial retrofits.
  • Up to 90% energy savings: Significant reductions in space heating and cooling costs.
  • Enhanced indoor air quality: Mechanical ventilation with heat recovery (MVHR) ensures a controlled, fresh, and comfortable air supply.
  • Superior thermal comfort: High-performance insulation and airtightness eliminate cold spots and drafts.
  • Efficient HVAC systems: Optimized heating, cooling, and hot water systems reduce energy consumption.
  • Lower life-cycle carbon emissions: Avoids “lock-in” effects where partial renovations leave high CO2 emissions unaddressed for years.

Step-by-step retrofits and partial renovations

For phased retrofits, buildings can receive pre-certification for all steps up to the final complete retrofit, under an EnerPHit Retrofit Plan (ERP). This ensures that when all phases are complete, the building meets EnerPHit standard. Pre-certification offers reassurance to owners and planners that performance targets will be achieved and helps spread the investment over a longer period.

EnerPHit Unit certification is also available for individual apartments within multi-residential buildings. This requires:

  • Airtightness verification: Either a pressure test (qe 50 ≤ 1.0 m³/(hm²)) or detailed documentation and photographic evidence of airtight construction.
  • Connection to adjacent spaces: Measures to ensure the retrofit works don’t generate moisture damage in neighbouring units.

Conclusions

EnerPHit offers several pathways to achieve Passivhaus certification. When carrying out an energy retrofit, it’s especially important to implement improvements in a way that avoids moisture damage. EnerPHit certification provides reliable and safe methodologies to avoid this, ensuring that existing buildings meet modern standards of efficiency and comfort, while significantly reducing their environmental impact.

Which Christmas tree is greener? Real, artificial, or potted?

A real tree which we then take to the recycling centre? A real tree, but potted, which we could then re-use every year. Or a plastic tree which we could re-use for several years.

Which Christmas tree is greener? Real, artificial, or potted?

We’re having family to stay in our new house for Christmas, and so I got talking to my daughter a few days ago about what we were going to do for a Christmas tree. Some years ago, we made a Christmas tree mobile, with sticks and leaves, which we decorate with LED lights and a choice selection of festive tinsel and Christmas ornaments. But what about pushing the boat out this year?

Imagen árbol de navidad

We ended up having an interesting, and fully woke discussion, about what would be the most environmentally friendly solution:

  • A real tree which we then take to the recycling centre?
  • A real tree, but potted, which we could then re-use every year (if we could keep it alive…).
  • Or a plastic tree (which we weren’t very keen on, but…) which we could re-use for several years.

So which option has the lowest carbon footprint? Here’s what the data says:

Artificial Tree: Manufacturing a 2 m PVC tree emits about 40 kg CO₂e. If you reuse it for 10+ years, its annual impact drops to ~4 kg/year, making it competitive (that is: 40 kg CO₂e spread over 10 years of use, means the average yearly emissions are 4 kg CO₂e/a …).

Real Tree (which is then composted): this option generates round 5 kg CO₂e per year. Composting or chipping is key to keep emissions low.

Real Tree (Landfilled): this is the worst option—up to 16 kg CO₂e per year due to the methane emissions from the rotting biomass (methane was a Global Warming Potential about 27 times higher than CO2).

Real Tree (Burned/Incinerated): this generated about 3.5 kg CO₂e per year, which is better than landfill, especially if it’s burnt in energy recovery facilities, where the heat is used for some other purpose.

Potted/Replantable Tree: This one is the winner in the long-term—roughly 20 kg CO₂e over 10 years if cared for and reused annually.

Annual CO2 Impact Comparison

Bottom line:

If you already own an artificial tree: keep using it as long as possible.

If you want a real tree: choose local, and compost or incinerate responsibly.

If you want the greenest choice, go for a potted tree which you can reuse or replant.

Sources:

  • Carbon Trust – Life Cycle Assessment of Christmas Trees
  • Zurich Insurance – Sustainability tips for festive season
  • ADEME (Agence de la Transition Écologique) – Environmental impact of natural vs artificial trees

Can Naiades: first summer in our Passivhaus…comfort, coolness and energy savings

I’ve worked on Passivhaus projects for more than a decade now, based here in Catalonia, north-east Spain. I came across the standard when I was doing a Masters in Architecture, Energy and Environmental studies at Centre for Alternative Technology in Wales (UK).

Can Naiades: first summer in our Passivhaus…comfort, coolness and energy savings

By Oliver Style, Praxis CEO

I’ve worked on Passivhaus projects for more than a decade now, based here in Catalonia, north-east Spain. I came across the standard when I was doing a Masters in Architecture, Energy and Environmental studies at Centre for Alternative Technology in Wales (UK). Passivhaus resonated with me…it made sense: to design, build, and retrofit buildings that are super comfortable, need very little energy and radically reduce CO2 emissions. So choosing a passive house was a way of living more coherently, and a personal statement of intent to fight against the climate emergency…of living better, with less.

It wasn’t until last year that I was able to take part in the design and construction of my own Passivhaus, Can Naiades, a prefabricated lightweight timber frame house located about 40 kilometres northeast of Barcelona, with a useful floor area of 128m2. Having heard from many clients about how wonderful it is to live in a Passivhaus, it’s quite different to experience it first-hand. What does it feel like? It feels solid, comfortable, and quiet. It feels safe, airy and light. It is everything I’ve never had in any of the houses I’ve previously lived in and complained about. It really is, GREAT!

Primer verano en nuestra Passivhaus

Surfing the heat waves

We moved in at the end of May 2025 and walked slap bang into the middle of a major heat wave, with average temperatures around 4 ºC higher than previous years and peaks of 37 ºC. Walking outside was like stepping into a furnace. We had no blinds for all of June and July (they were only installed in August), but despite that, it was wonderfully cool and comfortable. We did- of course- have our (one) air conditioning unit on quite a lot…but even so, our energy use from June-October was 3 % lower than predicted with the (calibrated) PHPP energy model. Fantastic!

The house has really worked a treat this first summer. Plenty of people complain that lots of insulation and airtightness means passive houses overheat in the summer. But, despite large amounts of glazing, Can Naiades has kept us nice and cool all summer, with 96% of our energy use coming directly from the solar PV panels and battery bank.

Temperature-wise, there is a noticeable difference between the ground floor (which has a big fat concrete floor slab with lots of thermal inertia), and the 1st floor (which has very little thermal mass). Heat rises of course, so to some degree that’s as expected…but a bit of thermal inertia really does help shave the peaks of those daily temperature swings.

Powered by the sun

Between June and October, we used only 137 kWh from the grid. We got our grid feed-in connection legalised at the end of September, so in October, 57% of the energy we generated with the PV panels we used in the house and pumped the remaining 43% into the grid…clean, fossil-fuel-free electricity.

And then we got our 1st energy bill: 19 € for the month of August, of which only 3€ was for the electricity we consumed from the grid (in total 18 kWh, or 0.15 €/m2). In the 80m2 flat we used to live in, we used 475 kWh the previous August and paid 95 € for a month of electricity (1.19 €/m2)…that’s 87% less (in €/m²). Bargain!

It’s a wrap!

I remember a friend telling me once, that the only problem with living in a Passivhaus is that you don’t sleep very well when you go and stay anywhere else! There are still a lot of jobs to finish in the house and we’re skint, so we’re not going very far for the foreseeable future…but I can confirm: living in a Passivhaus is a dream come true, especially in a Mediterranean summer!

But it shouldn’t be a luxury: it should be normal, and within everyone’s reach. In the context of a serious housing crisis in many European countries, decent, comfortable, and efficient homes should be accessible to the majority of the population- especially for low-income families, who often live in a situation of energy poverty. Both the public and private sector need to work to make this a reality.

For more tecnical information about the project, have a look at this article.

Can Naiades: professionals & contractors

Can Naiades: components and systems

  • Insulation: Panel Plus TP138, Smart Wall FKD-N Thermal, Knauf Insulation
  • Specialist insulation: Nanoboard Aerogel, Pafile
  • Timber structure: EGOIN
  • Windows: Smartwin Compact, Ventanas Gardea
  • Window subframes: ISO-TOP construction sheets WF3, Iso Chemie
  • Airtightness tapes & membranes: SIGA & Onhaus
  • Liquid airtight membrane and radon gas barrier: Soudatight SP & LQ, Soudal
  • Radon gas sensors: Bequerel
  • Control & monitoring system: Loxone
  • Rainwater catchment tank: Simop 6328
  • Grey water treatment system: Intewa Aqualoop, Ecospai 
  • Shading devices: Solomatic II 80 FIX, Griesser España
  • Rooflight: DEC-C U8 + AMZ/C Z-Wave awning blind, Fakro
  • Heat pump (heating, cooling, hot water): Aquarea Ecoflex, Panasonic
  • DHW heat recovery systems: Zypho iZi 30 & Zypho PiPe 65, Aliaxis
  • MVHR unit: Zehnder ComfoAir Q450 ERV + ComfoClime Q, Zehnder
  • Solar PV system: 21 TwinPeak5 410W PV panels; 1 Primo GEN24 8.0 Plus hybrid inverter; BYD B-Box Premium HVM 13.8kW battery bank, Prot Energia